This invention relates generally to pneumatic switches and more particularly, to a high gain pneumatic switch for detecting a very low pneumatic pressure and for generating an output signal at a pressure substantially in excess of the pressure detected.
In heating, ventilating and air conditioning systems used in modern buildings, air ducts are provided for drawing outdoor ambient air into the building, for exhausting air from the building to the outdoors and for air mixing. A typical air handling unit would include a plurality of fans disposed within the duct work for providing air movement. When designing the controls for such HVAC systems, the architect or engineer frequently finds it desirable to predicate the occurrence of a subsequent step in the control strategy upon the occurrence of a preceding step. For example, it may be desirable to energize electric heater strips disposed within the duct only if the appropriate fan is flowing air thereacross. While one may obtain a signal indicative of an operating fan from, for example, an auxiliary contact upon the fan motor starter, the most positive manner in which fan operation may be verified is by sensing the resulting air pressure within the duct. In yet other types of applications, it may be desirable to provide a switch for generating a modulated output signal which is generally proportional to a very low but modulating pressure within the duct. This latter circumstance may be desirable in HVAC systems which use variable speed fans.
In addition to having the capability of detecting and reacting to very low pressures within an air duct, it is often desirable to provide a switch which generates a pneumatic output signal at a pressure which is useful to directly position pneumatic cylinders and the like which may be incorporated in HVAC equipment for positioning dampers or louvers. Such pneumatic cylinders are widely available and suitable for actuation at pneumatic pressures in the 3-20 pounds per square inch gauge (psig) range.
One type of known pneumatic switch is used as a pressure signaling or limiting device and incorporates a sensing chamber having a resilient diaphragm for positioning a snap-acting electrical switch when a predetermined pressure is reached. Examples of such switches are shown and described in U.S. Pat. Nos. 3,178,531 and 3,433,910.
In another type of known switch, a low pressure pneumatic input signal is sensed by a chamber including an elastomeric, impregnated diaphragm confined between two rigid plates. Diaphragm and plate movement is transmitted to a control module by a lever coacting between the diaphragm and the module and pivotably supported at one end. The lever functions to manipulate a plunger and control diaphragm into sealing engagement with a nozzle for generating a pneumatic output signal. The point of contact of the plunger and lever is immediately adjacent the lever pivotable support point and the arrangement of the overall structure results in erratic output signals which impair the performance of carefully engineered systems. An example of such a pneumatic switch is shown in Product Bulletin P5232 of Johnson Controls, Inc.
In yet another type of pneumatic switch, two input pressures are sensed, one on either side of a resilient diaphragm. Movement of the diaphragm caused by a differential between the sensed pressures is used to provide an output signal, the value of which is proportional to the difference between the sensed input signals. The proportional output signal is generated by using the diaphragm to modulate a control nozzle and an example of a switch of this type is shown in U.S. Pat. No. 3,612,085. In a differential switch of another type, diaphragm movement is used to actuate an electrical switch upon the attainment of some predetermined, minimum differential pressure. An example of a switch of this type is shown and described in Bulletin No. 4145-A of Johnson Controls, Inc.
Yet another type of pneumatic apparatus which uses a movable diaphragm, a disk for modulating a flow passage and a lever coacting between the disk and the diaphragm is shown in U.S. Pat. No. 3,160,169. The device shown therein defines a pressure regulator and safety vent valve for use upon combustible gas lines and since it employs only a single source of gas pressure, is incapable of pressure amplification or of detecting and responding to pressures at very low levels.
While these devices have heretofore been generally satisfactory for detecting pressures in an HVAC system, they tend to be characterized by certain disadvantages. In particular, certain of them are capable of functioning as a differential switch, i.e., of reacting to a difference between two pressures sensed thereat. Other switches such as that shown in the aforementioned bulletin of Johnson Controls, Inc. are capable of responding to very low duct pressures but are adapted to provide an electrical output signal and are therefore wholly incapable of providing a pneumatic output signal directly, either of a digital type or of a modulated analog type.
A high gain pneumatic switch capable of sensing very low pressures and which is adapted to utilize a plurality of pressure sources for providing an input signal and for generating a high gain output signal which may be digital or analog in nature would be a distinct advance in the art.
It is an object of the invention to provide an amplifying pneumatic switch which is responsive to very low pressure signals.
Another object of the present invention is to provide a pneumatic switch which includes a chamber for receiving an input signal from a first pneumatic pressure source at a very low pressure.
Another object of the present invention is to provide a pneumatic switch which incorporates a nozzle sealable for generating a pneumatic output signal from a second pneumatic pressure source where the signal may be used to directly position a pneumatic cylinder.
Yet another object of the present invention is to provide an amplifying pneumatic switch which incorporates a modulating assembly coacting between the first chamber and the nozzle for output signal generation.
Still another object of the present invention is to provide a pneumatic switch wherein the output pressure is generally proportional to the input pressure. How these and other objects are achieved will become more apparent from the description thereof taken in conjunction with the accompanying drawing.